Our previous research has shown that sera from patients with halothane hepatitis recognize microsomal antigens in rat liver which consist of a metabolite of halothane, the trifluoroacetyl (TFA-) group, bound covalently to specific liver microsomal proteins. We have now found that these protein antigens constitute the major TFA-protein adducts produced in livers of rats exposed to halothane, and that the protein antigens which are the most commonly recognized by the patients' antibodies (100 kDa, 76 kDa, 59 kDa and 57 kDa) are apparently long-lived. Expression of the antigens was increased in rats treated with isoniazid or clofibrate, which induce specific isozymes of cytochromes P-450. This suggests that antigen expression in livers of halothane exposed humans may vary as a function of the hepatic cytochrome P- 450 isozyme profile, and that this may be one factor which pre- disposes certain individuals to development of halothane hepatitis. In addition, we found that the antigens can be solubilized from the microsomal membrane using low concentrations of nondenaturing detergents (0.1% deoxycholate or Triton X-100). This property has greatly facilitated our efforts to purify the antigens. Purification to homogeneity is the major goal of the research currently being undertaken. The purified antigens will be used for the detailed biochemical characterization of the proteins, in animal model studies, and as a source of antigen in ELISA methods for the sensitive detection of patients sensitized to halothane or other structurally related inhalation anesthetic.